Self-disclosure has two elements:
Breadth: disclose a lot about ourselves at the start of the relationship, but it is often superficial, low risk information that we would be willing to reveal to anyone (friends, colleagues, acquantances). Many topics are Off limit and you don't want to reveal too much too soon.
Depth: over time, disclosure becomes deeper. Remove more and more layers to reveal our true selves covering lots of different topics and reveal our inner- self. Intimate, high risk information, painful memories, powerful secrets, strongly held beliefs.